23:32 12 June 2007
NewScientist.com news service
David Shiga

The fastest flows of matter in the universe shoot out of dying stars at more than 99.999% the speed of light, new observations reveal.

When a massive star runs out of fuel, it collapses to form a black hole or a neutron star. In the process, some of the matter from the star also explodes outwards at blistering speeds, producing an intense burst of gamma rays and other radiation.

Scientists had predicted that the matter expanding in these explosions would be propelled to very nearly the speed of light, but it has previously not been possible to clock them precisely.

Now, rapid follow-up measurements of two gamma-ray bursts have allowed a team of scientists to precisely measure the expansion speed of matter in these explosion to more than 99.999% the speed of light. The team was led by Emilio Molinari of the Osservatorio Astronomico di Brera in Merate, Italy.

The expanding matter initially produces gamma rays, but when it starts colliding with surrounding gas, it creates afterglows in visible and infrared light. The amount of time it takes for this afterglow to reach its peak brightness can be used to calculate how fast material in the jets is moving.

The researchers used a robotic infrared telescope called Rapid Eye Mount (REM), based in La Silla, Chile, which quickly points at gamma ray bursts detected by NASA's Swift satellite.

Fastest flows
For two bursts in 2006, called GRB 060418 and GRB 060607 A, the telescope was able to start watching for the afterglow just one minute after Swift detected the burst. This allowed them to measure the delay before the afterglow peaks of 153 and 180 seconds, respectively. In both cases, the speed of the expanding matter worked out to about 99.9997% the speed of light.

Team member Stefano Covino, also of the Osservatorio Astronomico di Brera, says single particles, such as neutrinos, can move at even higher velocities. But the total amount of matter moving at such high speeds in the GRBs is equivalent to 200 times the mass of the Earth.

"There's no other organised flows of matter that go this speed," agrees Neil Gehrels, chief scientist for the Swift mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, and not a member of Molinari's team.

Slowing time
Gehrels says it is an important accomplishment to have measured this speed precisely for the first time. Although scientists expected the speed of matter expanding in gamma-ray bursts would be close to the speed of light, they were not sure exactly how high it would be.

The speed can be translated into something called a Lorentz factor, a number that describes how much time slows down for objects moving close to the speed of light. The matter expanding in the two gamma-ray bursts studied here had a Lorentz factor of about 400.

The measured speed fits nicely with the prevailing picture of gamma-ray bursts, called the fireball model, in which matter exploding outwards at high speeds produces the gamma rays and afterglows.

"Certainly the fireball model does predict this [Lorentz factor] number and it predicts that it should be greater than 100," Gehrels told New Scientist.

Journal reference Astronomy & Astrophysics (DOI: 10.1051/0004-6361:20077388)
http://space.newscientist.com/article/dn12051-jets-of-matter-clocked-at-nearlight-speed.html